The present invention relates to a data storage apparatus and a computer system using recording media such as an optical disc apparatus or a magnetic disc apparatus, which are connected with an interface system.
In recent years, SCSI (Small Computer System Interface) is generally used as an interface for small-size computer systems. "Newest SCSI Manual" (published by CQ Publishing Co., Ltd., Jan. 1, 1989, pages 146-147, 177) provides a detailed description of the SCSI system. The SCSI is mainly connected to a host system (host computer) and an external memory such as a magnetic disc apparatus or an optical disc apparatus or a magnetic tape apparatus. Since the SCSI is operated by protocols, it is easy to connect and disconnect the host system and the external memory to and from the SCSI. That is, it has high expandability and versatility.
SCSI accepts eight apparatuses (including a host system) to be connected, and ID (identifier:identification number) of 0 to 7 is allocated to each apparatus. When the apparatuses connected to the SCSI are operated, they must be classified into an initiator (apparatus for issuing command) and a target (apparatus for receiving command). Generally, the host system is an initiator and the external memory is a target, and the external memory performs operations according to a command from the host system. However, the external memory may become an initiator, or the host system may become a target.
An example of the operation of a copy command (data copy) and its protocol for using the SCSI bus shall be described using FIG. 14. The copy command carries out data backup from a magnetic disc apparatus 202 which contains a SCSI controller 212 to a optical disc apparatus 203 which contains a SCSI controller 213. FIG. 14 shows the construction of an SCSI apparatus. The magnetic disc apparatus 202 and the optical disc apparatus 203 each include a disk controller 222, 223, and the SCSI controller of each is connected separately with a SCSI bus 204.
The operation of the copy command is as follows. First, a host system A 200 issues a reserve command 241 to the magnetic disc apparatus 202, and also issues a reserve command 242 to the optical disc apparatus 203, and inhibits access from another host system B 201. Next, the host system A 200 issues a copy command 243 to the optical disc apparatus 203. In this case, a copy command 243, assigns parameters including the ID of a data source and output destination apparatus, the LUN (Logical Unit Number), the top address and the copy block number on the magnetic disc 232, and the data block length of the optical disc 233. The optical disc apparatus 203 operates as an initiator after receiving the copy command 243 with the parameters.
Thereafter, the disc apparatus 202 reads out data from its magnetic disc 232, and carries out data transfer 244 to the optical disc apparatus 203 through the SCSI bus 204. The optical disc apparatus 203 writes the transferred data. When the data transfer 244 is finished, the optical disc apparatus 203 carries out reconnect processing 245 and informs of the completion of the copy command to the host system A 200. Then the host system A 200 releases the reserve state 246.
In above described operation, the conventional data storage apparatus has the following problems.
The execution of copy and verify (data comparison) commands through the SCSI bus occupies the SCSI bus for a long time, and it causes degradation of the system performance. Also large overhead of the SCSI protocol causes degradation of throughput in the SCSI apparatus.
In addition, miniaturization of the magnetic disc apparatus is occurring rapidly due to recent developments, and a problem occurs in that all circuits including the SCSI controller cannot be mounted within a miniaturized apparatus.
Also attendant on miniaturization of the optical disc apparatus or the magnetic disc apparatus, low consumption of power must be maintained.
Also since data transfer speed of the optical disc apparatus or the magnetic disc apparatus becomes high, read/write data signal must be treated in parallel as long as possible. Further, a transfer cable of read/write data signal must be shortened.
On the other hand, in a data storage apparatus such as an optical disc apparatus or a magnetic disc apparatus, it is quite important that the written data are read out correctly. Consequently, the important technical problem of the recording and reproducing is to secure the data reliability. Therefore the data reliability is improved by active introduction of error correction code ECC or the like. In this case, comparing reliability of the optical disc apparatus and the magnetic disc apparatus, the optical disc apparatus normally has a higher bit error rate. Consequently, in the optical disc apparatus, the ECC correction capability is strengthened in comparison with the magnetic disc apparatus.
In the optical disc apparatus, when the sector length is 1024 bytes, a Reed-Solomon code is used in the ECC thereby burst error of 80 bytes is made capable of being corrected. On the other hand, in the magnetic disc apparatus, a fire code has been used in the ECC , but a Reed-Solomon code (11 bytes) with high ECC capability is being adopted due to the use of high density recording. In this case, burst error of 2 bytes can be corrected.
The optical disc apparatus or the magnetic disc apparatus progressively higher density recording thus carrying the reproduction of read data to become more difficult. Specifically, due to high density recording, the read-out signal from the recording medium is miniaturized and made high speed and the S/N becomes lower. As a result, phase synchronization clocks to discriminate the read data and write clocks become high speed and the reproducing margin of the read data is decreased. Particularly, in the magnetic disc apparatus, since a gap between the head and the recording medium is decreased due to the high density recording, the contact probability between both becomes high and the bit error rate becomes high.
For this reason, the conventional data storage apparatus has another problem. Since the ECC capability of the magnetic disc apparatus is low in comparison with the optical disc apparatus, the read data error generating probability becomes high, and a serious problem is produced in that the error recovery cannot be made in the conventional system.